Electric furnace



2 Sheets-Sheet l W. l. ROBERTSON ELECTRIC FURNACE Filed Aug. 8, 1950 HEEL Aug. 7, 1934.

Aug. 7, 1934. w. 1. ROBERTSON ELECTRI C FURNACE 2 Sheets-Sheet 2 Filed Aug. 8 1950 Patented Aug. 7, 1934 rarer orris ELECTRIC FURNACE William Iverach Robertson London, England Application August 8, 1930, Serial No. 473,948

In Great Britain February 7, 1930 1 Claim.

requiring similar temperatures.

The invention aims to provide'a compact and robust furnace for such work capable of giving the high temperatures required under complete control and with due economy of energy; to facilitate the removal of the article treated from the furnace, by enabling the operator to shift the article without shock or jarring to a relatively cool spot while he is himself protected from the heat of the furnace; and to simplify the construction and assembly of the furnace so that repair or replacement of damaged parts is made easy.

With these aims in view the invention comprises in one aspect an electrically heated furnace of the vertical tubular type in which there is a hearth filling the cross-section of the furnace and movable to the upper end of the tube, the hearth being guided in its movement so as not to touch the tube. By so lifting the hearth the article treater is carried up to the mouth of the furnace and at the same time the mouth of the furnace is closed by the hearth which is then interposed between the hot interior of the furnace and the operator.

The invention further comprises a vertical tubular furnace having a hearth filling the crosssection of the furnace and movable to its upper end to come flush with its mouth, and a table closely surrounding and flush with the mouth of the tube to which a fragile article may be transferred without the least risk of damage.

By employing a stout resistant conductor and operating the furnace at low voltage relatively long life of the conductor is ensured; nevertheless the conductor needs to be accessible for renewal notwithstanding the ample lagging about it necessary for economy of energy. Seen from this aspect the invention is a vertical tubular furnace heated by a resistant conductor wound about the tube and lagged by heat insulation confined between the tube and a surrounding casing, tube and easing entering recesses in a base and a table parallel with the base and forming distance pieces between the base and table which are drawn together by bolts.

Other features of the invention contributing to the making of a satisfactory dental furnace will appear from the following description of a preferred embodiment. This is illustrated in the accompanying drawings,

Figure 1 being an elevation of the furnace proper partly in setcion on the line I-I of Figure 2,

Figure 2 a plan of this part,

Figure 3 a plan of the furnace complete with its base and accessory electrical apparatus, and

Figure 4 a diagram of its electrical connections. 1

In this furnace the-heating chamber is a vertical tube 1 which is carried at a height above the bench or the base of the furnace by a frame 2 erected on'legs 3. On the frame 2 is a base plate 4 of refractory insulating material, for instance a composition of asbestos agglomerated bya mineral cement, such as those sold under the trade names Uralite and Sindanyo, and at a height above it and parallel to it is a top or table 5 of similar material. The tube 1 is inset into recesses in the base plate l and table 5, and'in the table there is an opening of the same diameter as and alined with the furnace bore. Around the heating chamber 1 and at a distance from it is a metal casing 6 which enters annular grooves in the upper face of the base plate 4 and under face of the table 5. The space between the chamber 1 and easing 6 is filled with lagging 7, preferably a heat insulating material in powder form, such as kieselguhr. The tube 1 and the casing having been set in position in the base plate 4 and the casing filled with lagging, the table 5 is placed upon the tube and casing and secured to the base 4 by means of bolts and nuts 8.

The lower end of the heating chamber is closed by a plug 9 of insulating material such as opaque quartz as well as by the base plate 4, and through both of these there is bored an axial opening.

Below the frame 2 there is attached a metal tube 10 alined with this opening. A quartz or other refractory rod 11 fits slidably in the tube 10 and the bore of the plug 9 and in base 4 and at its upper end carries a preferably detachable fiat topped hearth 12, which may also be of quartz, but is preferably of Alundum. This hearth substantially fills the cross-section of the heating chamber but is kept from actual contact with the walls of the chamber by the guidance of the rod 11. When the furnace is in use the rod 11 rests upon a metal block 13 of such height that the hearth is supported at about the middle of the heating chamber, said block assisting to keep the end of the rod 11 cool.

The tube 1 is made of insulating material, such as corundum or the artificial varieties thereof sold under the trade name Alundum and heated by a resistant wire. Such wire may be zigzagged up and down the inner furnace Wall within quartz tubes, or more conveniently wound in a helical groove in the substance of the insulating material. In the drawings such a groove is shown upon the outer surface of the chamber 1. The Wire may be held. in the groove by finer refractory wires passed around it and through holes in the wall of the chamber; or preferably perforated lugs oreyes 14 are formed upon the chamber at the ends of the helical groove and the resistant wire 15 is itself passed through them. The ends of the resistant wire are reinforced by doubling back and are joined to stout conductors 16 andby them connected to terminals 17. V

For the baking of porcelain crowns a high temperature, of the order of 2500 F., may be required and the resistor as well as the insulating material must be capable of withstanding this temperature. A platinum-iridium alloy will serve the purpose, but an alloy of rhodium and platinum containing about 10% of rhodium has so far been found to give maximum life at these high temperatures.

Particularly where alternating current is available it is preferable to employ a fairly stout resistor 15 and to supply it at low voltage by the use of a transformer. A transformer for this purpose together with switch and other control apparatus may conveniently be accommodated in a box-form base 18 having a top of refractory material, such as Sindanyo and sides of perforated metal built upon a metal frame. The top forms a convenient work bench for use in connection with the furnace and in front has a sloping panel 19 accommodating a switch 20 and ammeter 21. The transformer 22 has an intermediate tapping 23 (Fig. 4) on its low tension secondary winding for the purpose of enabling the furnace to be run ata low range of temperatures as well as at the high temperatures above suggested. An adjustable resistance 24 controlled by a slider 25 is mounted in the base and affords a means of exactly controlling the temperature of the furnace. As appears from Figure 4 the terminals of the primary winding of transformer 22 are connected with a plug 26 or other means of conveniently connecting the apparatus to supply mains. One terminal of the secondary winding of the transformer is joined direct to one of the terminals 17 of the furnace resistor. Either the tapping 23 or the other end of the secondary winding may be joined by switch 20 through the ammeter 21 and adjustable resistance 24 to the other terminal 17. The switch 20 has three positions, one for high temperatures, one for low and an intermediate off position.

The heating chamber 1 while in use is closed at its top, the closure being either attached to and movable with'the hearth 12, or consisting as shown of a separate plug 27 of quartz, Alundum or the like, having a bail or other means by which it may be readily lifted. This plug is formed to rest upon the table 5 and when removed may be placed in a hole 28 in the top of the box-form base 18 provided for the purpose. The cover having been removed, the rod 11 can be lifted and the hearth 12 brought flush with the table 5 whereupon the article treated may be readily shifted on to the table. As the hearth 12 closes the mouth of the furnace the operator handling the article is not exposed to its radiant heat.

What I claim is:'

Invan electric furnace the combination of a base plate and a parallel table, a casing separating and fitting into grooves in said base plate and table, a vertical tubular heating chamber of insulating material of substantially less diameter than said casing fitting into the centre of said base plate and table, a refractory resistant conductor wound about said chamber, lagging filling the space between said chamber and casing, said table having an opening of the internal diameter vertically in said chamber.

,of said chamber, and a hearth freely movable 

